Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying a land surface or seafloor. Among other things, seismic data acquisition involves generating acoustic waves and collecting reflected/refracted versions of those acoustic waves to generate an image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process in the field of seismic surveying.
Mapping subsurface geology during exploration for oil, gas, and other minerals and fluids uses a form of remote sensing to construct two-dimensional or three-dimensional subsurface images. The process is known as seismic surveying, wherein an energy source transmits pressure pulses into the earth. These pressure pulses can be reflected by geological interfaces associated with the earth and subsequently recorded at the surface by arrays of detectors (receivers).
In order to provide some context for the process of seismic acquisition, consider a seismic data acquisition process and system as will now be described with respect to FIG. 1, in which a vessel 110 tows plural detectors 112, which are disposed along a cable 114. Cable 114 together with its corresponding detectors 112 are sometimes referred to, by those skilled in the art, as a streamer 116. Vessel 110 may tow plural streamers 116 at the same time. The streamers may be disposed horizontally, i.e., lie at a constant depth z1 relative to the ocean surface 118. Also, plural streamers 116 may form a constant angle (i.e., the streamers may be slanted) with respect to the ocean surface.
Still with reference to FIG. 1, vessel 110 may also tow a seismic source array 120 configured to generate an acoustic wave 122a. The acoustic wave 122a propagates downward and penetrates the seafloor 124, eventually being reflected by a reflecting structure 126 (reflector R). The reflected acoustic wave 122b propagates upward and is detected by detector 112. For simplicity, FIG. 1 shows only two paths 122a corresponding to the acoustic wave. Parts of reflected acoustic wave 122b (primary) are recorded by the various detectors 112 (recorded signals are called traces), while parts of reflected wave 122c pass detectors 112 and arrive at the water surface 118. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), reflected wave 122c is reflected back toward detector 112 as shown by wave 122d in FIG. 1. Wave 122d is normally referred to as a ghost wave because it is due to a spurious reflection. The ghosts are also recorded by detector 112, but with reverse polarity and a time lag relative to primary wave 122b. Recorded traces may be used to determine the subsurface (i.e., earth structure below surface 124) and the position and presence of reflectors 126.
While FIG. 1 shows a streamer having a flat shape, it is possible to have a streamer with a variable-depth configuration as illustrated in FIG. 2. The variable-depth profile may have any shape. One example of a variable-depth profile is defined by three parametric quantities, z0, s0 and hc. Note that not the entire streamer has to have the curved profile. The first parameter z0 indicates the depth of the first detector 254a relative to the water surface 258. The second parameter s0 is related to the slope of the initial part 252a of the streamer 252 relative to a horizontal line 264. The FIG. 2 example has initial slope s0 equal to substantially 3 percent. Other values may be used. Note that the streamer 252 profile in FIG. 2 is not drawn to scale because a 3 percent slope is relatively slight. The third parameter hc indicates a horizontal length (distance along the X axis in FIG. 2 measured from the first detector 154a) of the curved portion of the streamer. This parameter may be in the range of hundreds to thousands of meters.
The traditional acquisition system 300 illustrated in FIG. 3 (which corresponds to the system shown in FIG. 1) tows both the source array 320 and the streamer spread 330 (which may include streamers 316 and/or source arrays 320) along a same survey line 340. Thus, azimuth diversity (where the azimuth is defined as the angle α made at a receiver 312 between the streamer direction (along the inline direction X), and an imaginary line 332 connecting source array 320 with receiver 312) is very narrow for this traditional configuration. To improve azimuth diversity, more modern seismic surveys use an additional vessel 350 that tows only source arrays 352 along a source line 360, which is parallel but offset in a cross-line direction Y from survey line 340. In this way, azimuth angle α′ and data quality are increased. Note that for traditional seismic surveys, vessel 310 passes survey line 340 a single time.
As vessel 310 passes over the subsurface of interest, the source array generates pulses on a predefined schedule, and the streamer's receivers record the corresponding pressure waves and/or particle motion data. How the source arrays and receivers are positioned during the seismic acquisition campaign strongly influences collected seismic data quality, and also the time necessary to survey the desired subsurface. Experience has shown that the traditional seismic configurations illustrated in FIGS. 1 and 3 are not optimal. For example, not having the source arrays and/or receivers positioned according to a best scheme may increase the survey's overall duration, which negatively affects the time available for source array maintenance, increases exposure of survey personnel to health, safety and environmental (HSE) hazards, and induces higher survey costs.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks associated with wide-azimuth seismic data collection and provide a source array/receiver configuration, a source firing schedule or a tow vessel path configuration that improves target illumination, records near offset data with fewer passes and/or less time, provides a greater fraction of time for source array maintenance and shooting plan flexibility, while also minimizing interference and background noise collected with the seismic data.